Effects of anatomical differences on electromagnetic fields,SAR, and temperature change

Electromagnetic field simulations are increasingly used to assure RF safety of patients during MRI exams. In practice, however, tissue property distribution of the patient being imaged is not known, but may be represented with a pre‐existing model. Repeatedly, agreement in transmit magnetic ( ) field distributions between two geometries has been used to suggest agreement in heating distributions. Here we examine relative effects of anatomical differences on distribution, specific absorption rate (SAR), and temperature change (ΔT). Numerical simulations were performed for a single surface coil positioned adjacent a homogeneous phantom and bovine phantom, each with slight geometric variations, and adjacent two different human body models. Experimental demonstration was performed on a bovine phantom using MR thermometry and mapping. Simulations and experiments demonstrate that distributions in different samples can be well correlated, while notable difference in maximum SAR and ΔT occur. This work illustrates challenges associated with utilizing simulations or experiments for RF safety assurance purposes. Reliance on distributions alone for validation of simulations and/or experiments with a sample or subject for assurance of safety in another should be performed with caution. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 46B: 8–18, 2016     

Impact of chronic inflammation on the pharmacokinetic–pharmacodynamic relationship of naproxen

Objectives The use of biomarkers for predicting the clinical doses of analgesic drugs relies on the understanding of the relationship between drug exposure and response under disease conditions. In this study, we demonstrate the relevance of such a relationship for COX‐inhibitors by modelling the effect of naproxen on prostaglandin E2 and thromboxane B2 in a chronic inflammation model in rats. Methods: Rats were treated with Freund's complete adjuvant (FCA) by intraplantar injection. On post‐inoculation days (PID) 7–21, animals received single or chronic (qd until day 21) doses of naproxen (10 mg/kg). Blood samples were collected at various intervals after dosing to characterise naproxen pharmacokinetics and its effects on and production. PK‐PD modelling was performed using nonlinear mixed effects in NONMEM. Results: The inhibition of and could be described by a sigmoid model. A decrease in the potency estimates of both biomarkers was observed under chronic inflammation, as compared to healthy animals. values for inhibition showed a shift from to , whilst values for inhibition increased from to in healthy and FCA‐inoculated animals, respectively. Conclusions: Our results show that chronic inflammation causes a significant change in the potency estimates for COX‐inhibition. These findings illustrate the implications of pathophysiological processes on pharmacodynamics and consequently on the required exposure levels for achieving response during chronic treatment.     

Dependence of and field patterns of surface coils on the electrical properties of the sample and the MR operating frequency

In high field MRI, the spatial distribution of the radiofrequency magnetic ( ) field is usually affected by the presence of the sample. For hardware design and to aid interpretation of experimental results, it is important both to anticipate and to accurately simulate the behavior of these fields. Fields generated by a radiofrequency surface coil were simulated using dyadic Green's functions, or experimentally measured over a range of frequencies inside an object whose electrical properties were varied to illustrate a variety of transmit ( ) and receive ( ) field patterns. In this work, we examine how changes in polarization of the field and interference of propagating waves in an object can affect the spatial distribution. Results are explained conceptually using Maxwell's equations and intuitive illustrations. We demonstrate that the electrical conductivity alters the spatial distribution of distinct polarized components of the field, causing “twisted” transmit and receive field patterns, and asymmetries between and . Additionally, interference patterns due to wavelength effects are observed at high field in samples with high relative permittivity and near‐zero conductivity, but are not present in lossy samples due to the attenuation of propagating EM fields. This work provides a conceptual framework for understanding spatial distributions for surface coils and can provide guidance for RF engineers. © 2016 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 46B: 25–40, 2016